The invention relates to the manufacture of rotary drill bits for use in drilling or coring deep holes in subsurface formations.
The invention is applicable to rotary drill bits of the kind comprising a bit body having a shank for connection to a drill string, a bit face on the bit body, a plurality of cutting structures mounted in sockets in the bit body and projecting from the face of the bit, and a number of nozzles also mounted in sockets in the bit body and communicating with a passage for supplying drilling fluid to the face of the bit.
Each cutting structure may comprise a cutting element mounted on a carrier, such as a stud or post, which is received in a socket in the bit body. One common form of cutting element comprises a circular tablet having a hard facing layer of polycrystalline diamond or other superhard material and a backing layer of less hard material such as cemented tungsten carbide.
Rotary drill bits of this kind are commonly formed by one of two basic methods. In one method, the bit body is formed by a powder metallurgy process. In this process a hollow mould is first formed, for example from graphite, in the configuration of the bit body or a part thereof. The mould is packed with a powdered matrix-forming material, such as tungsten carbide, which is then infiltrated with a metal alloy, such as a copper alloy, in a furnace so as to form a hard matrix. In order to form the sockets to receive the cutting structures, it is usual for formers, also for example of graphite, to be mounted on the interior surface of the mould before it is packed with tungsten carbide. After the bit body has been formed the formers are removed and the carriers of the cutting structures are located and secured within the resulting sockets. Bit bodies formed by this process have the advantage of being highly resistant to erosion during use, due to the hardness and wear resistance of the matrix material. One problem with such method however, is that it is extremely difficult to control to a great degree of accuracy the size, location and orientation of the sockets in the bit body and this may lead to difficulties in fitting the cutting structures within the sockets. Resulting inaccuracies in the orientation of the cutting structures may also have a deleterious effect on the performance of the bit.
In an alternative method of construction, the bit body is machined from a solid blank of machinable metal, usually steel. Since the sockets are then formed in the bit body by machining it is possible to determine their size, location and orientation with great accuracy, for example by using computer controlled machining tools. However, the bit face of a steel-bodied bit is susceptible to wear and erosion during use, particularly in the vicinity of the cutting structures and of the nozzles from which drilling fluid emerges at high velocity and with substantial turbulence. Accordingly, attempts have been made to increase the wear-resistance of steel-bodied bits by applying a hard facing to the bit face, around the cutting structures. Various hard facing materials and methods have been employed but all suffer from certain disadvantages.
It would therefore be desirable to combine the accuracy of manufacture of steel bodied bits with the erosion resistance of matrix bits, and the present invention sets out to achieve this.